One day ahead:Make the détrempe by whisking the dry ingredients together in a mixing bowl. Pour in milk, water, and butter. Use the paddle attachment on a mixer to combine on the lowest speed for 1 + 1/2 minutes. Increase speed to medium-high and continue mixing for 10 to 15 seconds. If dough is too wet, add flour; too dry, add water. Dough should be slightly tacky and supple. Transfer to a lightly floured work surface and form into a ball. Place in a lightly oiled bowl and cover in plastic wrap. Refrigerate overnight.

Baking day:

Cut cold butter into 16 pieces. Use a hand mixer to combine butter with flour in a medium mixing bowl. Once mixture is smooth, turn out onto a piece of lightly oiled parchment paper. Spritz with cooking spray and then cover butter block with a piece of plastic wrap. Use a pastry scraper or spatula to form into a square under the plastic wrap. The block should be 1/2–inch thick and smooth. Take care to keep butter cool. If it begins to warm up, place in refrigerator, plastic wrap and all.

Prepare a generously floured work surface at least 32 inches wide to roll out dough. Remove détrempe from refrigerator and place on surface, sprinkling flour on top. Roll the dough out to 12 ½ inches wide and 6 ½ inches long. Dough should be ½-inch thick.

Remove plastic wrap from butter. Turn over onto the left half of the dough, with a ¼-inch border on the left, top, and bottom. Pick up the right edge of the dough and fold over to meet the left. Crimp the edges on all four sides to seal. Roll the dough out again to form a ½-inch-thick rectangle 16 inches by 9 inches.

To laminate the dough: Roll the dough out again to form a ½-inch thick rectangle 16 inches by 9 inches. Fold the dough in thirds, letter-style. Transfer to a sheet pan and cover loosely with plastic wrap. Let rest for twenty minutes. Repeat the process—rolling out the dough, folding it into thirds, and then letting it rest twenty minutes at room temperature—two more times. Refrigerate for the final resting period.

Return dough to floured surface. Roll out to a rectangle 24 inches wide by 9 inches long. Use your pastry scraper to mark the dough at 4-inch intervals along the top. Do the same along the top, starting your marking 2 inches from the left edge. Cut from bottom left corner up to the first top notch, then back down to the next bottom notch, until the dough has been fully segmented into triangles, 4 inches wide by 9 inches tall. (There will be scraps, which can be formed into mini-croissants.)

In the base of each triangle, make a 1-inch slice in the middle. Spread the slice as far apart as possible, then use the “flaps” to roll the triangle up toward its point, forming a crescent roll.

Line a sheet pan with lightly oiled parchment paper. Place the croissants 1 ½ inches apart on the pan and use fingers to lightly curl the points of each roll inward. Tuck the nose of each roll down to the pan to anchor it. Cover with plastic wrap and let proof at room temperature for 2 ½ hours.

Preheat oven to 450º F for 20 minutes before baking. If using egg wash, brush delicately onto croissants now. When ready to bake, place pan in oven (plastic wrap removed) and lower temperature immediately to 375º F. Bake for 15 minutes, then rotate pan. Bake for another 15 to 20 minutes until croissants are completely golden. If the rolls are becoming dark too quickly, lower the heat to 325º F and extend cooking time as needed.